(1) Field of the Invention
The present invention relates to a scanning optical system, such as a color or monochrome scanner, a laser printer, or a laser plotter, which causes a light beam to optically scan an original placed on a scanning plane for reading image information from the original or recording a required pattern on a photosensitive material on the scanning plane. More particularly, the invention relates to a method and apparatus for detecting the shape of a beam spot on the scanning plane.
(2) Description of the Related Art
It is known that, with this type of scanning optical system, a deviation from a target value of the shape of a beam spot on the scanning plane due to an error in its optics or the like results in blurred edges of a recorded pattern or in scanning line separation (i.e. formation of a gap between adjacent scanning lines). To cope with such a situation, the following methods have been proposed and employed for detecting the shape of a beam spot on the scanning plane.
One of these methods is described with reference to FIG. 1. A drum 1 rotatable at a constant speed has a peripheral wall defining two slits 2 and 3 inclined 45 degrees in opposite directions with respect to a direction of rotation. A light beam is projected from outside to the drum 1 at right angles to the peripheral wall. A light sensor 4 is mounted inside the drum 1 to be opposed to a beam spot BS formed on a peripheral surface of the drum 1.
As shown in FIG. 2(a), when the first slit 2 traverses the elliptic beam spot BS, the light sensor 4 outputs a detection signal as referenced Sx. The time required for the first slit 2 to traverse the beam spot BS is obtained by counting clock pulses CK over a period (t1-t0) during which the detection signal Sx exceeds a predetermined level. Assuming that the count is Nx, the span of the beam spot BS in the direction X (in the direction perpendicular to the first slit 2) is derived from Nx by sin 45.degree.. Similarly, as shown in FIG. 2(b), the span of the beam spot BS in the direction Y (in the direction perpendicular to the second slit 3) is derived from Ny by sin 45.degree..
A second method is described in Japanese Patent Laying-Open Publication No. 64-13514, for example. According to this method, slits are provided at positions equivalent in optical distance to a scanning plane, and the light beam is projected to traverse these slits. The shape of a beam spot BS is determined by detecting light passing through the slits. More particularly, this method uses a first slit inclined with respect to a scanning direction of the light beam, and a second slit extending perpendicular to the scanning direction, the time required for the beam spot to traverse each slit being measured to obtain beam spot diameters in directions perpendicular to the respective slits.
A third method directly measures a beam spot size from an enlarged image of the beam spot formed on a two-dimensional image pickup device such as a CCD.
However, the known methods described above have the following disadvantages:
The first and second methods using two slits to determine the beam spot size, in short, obtain a parallelogram ABCD having the same inclination as the first and second slits and circumscribing the beam spot BS as shown in FIG. 3, and derive the shape of the beam spot BS from distances X and Y between the opposed pairs of sides. The sides BC and AD are parallel lines determined by the first slit 2 shown in FIG. 2(a), for example, whereas the sides AB and DC are parallel lines determined by the second slit 3 shown in FIG. 2(b).
The beam spot on the scanning plane normally has an elliptic shape, and there are an infinite number of elliptic shapes, instead of being just one, that inscribe the parallelogram ABCD. As shown in a two-dot-and-dash line in FIG. 3, for example, a beam spot BS' also inscribes the parallelogram ABCD. In other words, the first and second methods are liable to an error of detecting the beam spots BS and BS' of different shapes as having the same shape. It is thus impossible to correctly recognize elliptic shapes of beam spots. The beam spots BS and BS' in FIG. 3 have different widths in the secondary scanning direction. Although, as shown in FIG. 4(a), proper scanning and recording results may be secured with the light beam BS, the beam spot BS' could produce gaps between adjacent scanning lines as shown in FIG. 4(b).
The third method enables correct recognition of elliptic shapes, but has the drawbacks of involving irksome adjustment of the optics for projecting an enlarged image of the beam spot and of requiring a large and complicated apparatus. In addition, the influences of blooming and crosstalk must be taken into account.